An occupant protection system installed in a vehicle uses an acceleration sensor to detect that the vehicle has been subjected to a collision load from a vehicle front and cause an occupant protection device control unit to control the occupant protection system in accordance with a detection signal from the acceleration sensor. The occupant protection device control unit is a unit that is housed in a casing and installed in the front section of the vehicle body. Some of occupant protection device control units have an acceleration unit integrated thereinto, which are called a sensor integrated type. Integration of sensors allows the simplification of, for example, a signal system of the occupant protection system. Such a casing that houses an occupant protection device control unit is known by, for example, Japanese Unexamined Patent Application Publication Nos. 2002-308021 and 2010-083382.
The casing for an occupant protection device control unit known by Japanese Unexamined Patent Application Publication No. 2002-308021 is composed of a main body made of aluminum die-cast having its bottom surface opened and a cover made of a steel plate that covers the bottom surface of the main body. A substrate for the occupant protection device control unit is secured to the main body with screws, together with the cover located below the substrate. The main body has a mounting bracket formed thereon. The mounting bracket has a weak portion. The weak portion is formed by thinning a base end of the main body which provides a connection between the main body and the mounting bracket. Alternatively, the weak portion is formed by making a through-hole in the main body close to a base end of the mounting bracket.
The casing for an occupant protection device control unit known by Japanese Unexamined Patent Application Publication No. 2010-083382 is composed of a resin-made main body having its bottom surface opened and a cover that covers the bottom surface of the main body. A substrate for the occupant protection device control unit is secured to the main body with screws, together with the cover located below the substrate. The main body has a mounting bracket formed thereon. The mounting bracket has a weak portion. The weak portion is formed by making a bundle in the mounting bracket.
For casings disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-308021 and 2010-083382, a collision load to which a vehicle body is subjected is transmitted to the main body through the mounting bracket and to the substrate through screws and then to an acceleration sensor installed on the substrate. In addition, if an excessive collision load is exerted on the bracket through the vehicle body, the weak portion is torn off, thereby protecting an occupant protection device control unit inside the casing.
It is preferable that the collision load is transmitted from the vehicle body to the acceleration sensor as directly as possible without the main body in a transmission path in order to allow the collision load to be transmitted to the acceleration sensor more quickly, exactly. To that end, it is conceivable that the cover is changed to a base made of a steel plate or other metallic plate, and that a bracket is attached to such a base and mounted on the vehicle body. Such a casing for housing an occupant protection device control unit is known by Japanese Unexamined Patent Application Publication No. 2010-215127.
The casing for housing an occupant protection device control unit known by Japanese Unexamined Patent Application Publication No. 2010-215127 is composed of a main body made of aluminum die-cast or resin having its bottom surface opened and a base made of a metallic plate that covers the bottom surface of the main body. A substrate for the occupant protection device control unit is secured to the main body with screws, together with the base located below the substrate. The base has an integral mounting bracket formed thereon. The main body has an independent mounting bracket fixed thereto in a fragile manner. In other words, such an independent bracket is a member separate from the main body.
For a casing disclosed in Japanese Unexamined Patent Application Publication No. 2010-215127, a collision load to which a vehicle body is subjected is transmitted from the vehicle body to the base through the integral bracket and to the substrate through screws and then to an acceleration sensor installed on the substrate. In addition, if an excessive collision load is exerted on the bracket through the vehicle body, the independent bracket is torn off from the main body, thereby protecting an occupant protection device control unit inside the casing.
However, the casing disclosed in Japanese Unexamined Patent Application Publication No. 2010-215127 needs many components since the independent bracket is fabricated using members separate from the main body, resulting in an increase in cost.
The inventors found the followings. To avoid such a cost increase, it is conceivable that the bracket is formed only on the base made of a metallic plate in an integral manner, namely, an integral bracket is formed. If such an integral bracket is used, a weak portion may be provided on the integral bracket.
However, a base or an integral bracket made of, for example, a steel plate, has a relatively large ductility. For this reason, for a weak portion just provided on such an integral bracket, there is a limit on setting a lower level of breaking load for breaking the weak portion. In order to improve the protection of the occupant protection device control unit housed in the casing against a collision load, the weak portion must break quickly and reliably without plastic deformation before the weak portion receives a large breaking load.